1. Field of the Invention
The present invention relates to the art of optical applications including optical information processing, optical computing, optical conversion, and optical measurement, and more particularly to board-to-board optical interconnections for optically interconnecting electronic circuit boards in computers or electronic devices.
2. Description of the Relevant Art
As electronic computer and semiconductor technology is rapidly advancing in recent years, the processing speed of central processing units (CPUs) is increasing and semiconductor memories are becoming higher in density and integration. At the same time, there have been certain limitations posed by the nature of electric circuitry itself on interchip interconnections, chip-to-chip interconnections, and board-to-board interconnections. Specifically, the phenomena of electric interference and electromagnetic induction present fundamental obstacles to efforts to design interconnections for higher density.
Optical information transmission, on the other hand, has a much greater possibility for higher-density interconnections as it is entirely free from the above problems.
In view of the advantages of optical information transmission, there have been various approaches to the development of optical interconnections. For details of one known attempt, reference should be made to Conference Record of 1990 International Topical Meeting on Optical Computing, pages 162, 164, and 408, for example.
FIGS. 21 and 22 of the accompanying drawings show a conventional board-to-board interconnection proposed in the above paper.
As shown in FIG. 21, sockets each having an array of light-emitting and -detecting elements are mounted on a glass substrate (motherboard), and electronic circuit boards are connected to the glass substrate through the sockets. As shown in FIG. 22, light is propagated along a zigzag path in the glass substrate, allowing a signal to be transmitted from one socket another. The light-emitting and -detecting elements are optically coupled to the propagated light through a hologram element (HOE). The hologram element includes focusing lenses and a mirror or beam splitter to prevent the light from being spread due to diffraction when it is propagated along the zigzag path in the glass substrate.
The conventional board-to-board interconnection can be manufactured by the planar fabrication process because all the optical devices can be mounted on the glass substrate. However, it is highly difficult to reduce the aberration of the HOE lenses particularly for the reason that they focus the light off-axis, and the hologram element tends to produce unwanted diffracted light as noise.
Accordingly, the above prior optical signal transmission arrangement makes it difficult to achieve optical interconnections for higher resolution with good signal-to-noise ratio, or higher density.
Other problems are that inasmuch as the light is propagated along the zigzag path in the glass substrate, the glass substrate should have parallel surfaces with high precision, it is difficult to shorten the total length of the propagation path, and the time lag between the electronic circuit boards, such as a clock skew, is possibly not negligible.
Generally, an optical system comprising lenses, gratings, prisms, etc. suffers a drawback in that it requires an extremely complex process to align the optical axes of the optical components used. It is very difficult to position, highly accurately, various optical parts or optoelectronic parts, e.g., a spatial light modulator (SLM), a detector array, an light-emitting-diode (LED) array, a semiconductor laser (LD) array, etc., with respect to each other.
Furthermore, since mechanisms for aligning/fixing these parts are complicated, the entire optical system cannot be greatly reduced in size, and the positional stability or reliability of the assembled parts against temperature changes or other environmental changes cannot be maintained at a desired level.
Recent years have seen frequent exchanges of optical information between a plurality of parallel optical paths (see, for example, Conference Record of 1990 International Topical Meeting on Optical Computing, No. 10B2, 9D17, 10 Hz).